The present invention relates to a target member for producing Co--Ni--Cr magnetic thin films for magnetic recording by a sputtering method, and to a method of producing such a target member.
Magnetic thin films of Co--Ni, Co--Ni--Cr, Co--Cr, etc. are produced by a sputtering method or a vapor deposition method. In the case of a sputtering method, a target member is usually made of an alloy having substantially the same composition as that of a magnetic thin film to be produced.
Target members of Co--Ni, Co--Ni--Cr, Co--Cr. etc. are conventionally produced by casting, and the cast products are cut to thin plates or their cast ingots are subjected to hot working to provide thin plates.
80 Co--20 Ni magnetic thin films for hard disks, typical examples as thin films of in-plane magnetization, are produced from targets having substantially the same compositions, and the targets themselves are produced by casting: or by hot or cold working of cast products.
On the other hand, thin films of in-plane magnetization consisting of Co--Ni--Cr, typically 63 Co--30 Ni--7 Cr were recently proposed (Japanese Patent Laid-Open No. 61-120330), and have been getting much attention.
The magnetic thin films consisting essentially of Co, Ni and Cr are superior to the conventional magnetic thin films of 80 Co--20 Ni in respect to magnetic properties such as a coercive force and a corrosion resistance. Accordingly, they have been increasingly substituting for 80 Co--20 Ni.
In the case of 80 Co--20 Ni targets, the transformation from a .gamma.-phase to an .epsilon.-phase takes place in the step of cooling from high temperature in the process of producing such targets, because they contain 70 weight % or more Co. Accordingly, they are less magnetized in a low magnetic field, meaning that they have a magnetic flux density B of at most 7000 G or so at a magnetic field intensity H=1000 Oe and a low permeability.
On the other hand, in the case of 63 Co--30 Ni--7 Cr, an austenite structure appears by cooling from high temperature, and it shows a magnetic flux density B of about 10900 G at H=1000 Oe, and its maximum permeability is as large as 100 or more.
At present, most of sputtering apparatuses employ a magnetron system. In this magnetron system, a permanent magnet is placed just under a target member, and leakage magnetic field is generated just above the target member to trap electrons, thereby increasing the efficiency of the sputtering.
To generate this leakage magnetic field, an Alnico magnet or a rare earth magnet is used, but if the target member has a large permeability, magnetic flux passing through the target member increases, leading to the decrease in leakage magnetic field which contributes to the generation of plasma.
For this reason, in the case of a 63 Co--30 Ni--7 Cr target member, leakage magnetic flux for generating a plasma cannot be obtained without reducing the thickness of the target member. However, the thinner the target member, the shorter its service life, making It necessary to exchange the target member more frequently. In addition, the thinner the target member, tha more susceptible it is to partial erosion by sputtering, lowering the productivity of sputtered thin films.
Accordingly, consideration is being given to the reduction of permeability of a target member of this kind.
Various proposals have been made so far to achieve the above object. For instance,
(a) A target member is subjected to a cold working treatment such as cold rolling, surface cutting, rough grinding, etc. to reduce its permeability (Japanese Patent Laid-Open No. 61-113759). PA1 (b) A target member is produced in a porous form by a powder metallurgy method to reduce its permeability (Japanese Patent Laid-Open No. 63-24060). PA1 (c) A target member is produced in such a structure that non-magnetic phases of Ag, NbC, etc. are dispersed so that they can function as magnetic gaps which generate leakage magnetic flux on a surface of the target member [Japanese Patent Laid-Open No. 63-118067). PA1 (d) A conventional, typical composition of 63 Co--30 Ni--7 Cr is changed to 70.9 Co--20.2 Ni--8.9 Cr to reduce its permeability (Summary of The 10th Lecture of Japan Applied Magnetic Association, November 1986, p. 121).
However, the above methods have their oWn problems. Specifically, in the method (a), when the level of cold working is increased to sufficiently reduce the permeability, strain introduced by cold working i$ increased, and it is released when the temperature of the t member is elevated by brazing thereof to a backing plate, causing the target member to be warped. As a result, good brazing cannot ba achieved, and a low permeability cannot be obtained stably by this method.
In the method (b), the resulting target member does not have sufficient mechanical strength because it is porous. Accordingly, it has a problem in handling.
In the method (c), the resulting target member contains non-magnetic phases of Ag, NbC etc. which are not made of elements essential for the target member. Accordingly, when this is subjected to sputtering to produce a magnetic thin film, necessary magnetic properties are not likely to be obtained.
In the method (d), the resulting target member is likely to have slightly deteriorated magnetic properties.
As described above, in has been difficult to reduce a permeability of a Co--Ni--Cr target member effectively by any of the conventionally proposed methods.